Gas turbine engines such as the type used in aircraft generally include a compressor, a combustor, and a high pressure turbine in serial flow relationship. Air entering the engine is compressed by the compressor and then mixed with fuel and ignited to form a high energy gas stream. The gas stream passes through the high pressure turbine where some of the combustion energy is extracted to drive the compressor. Turboprop and turbofan engines used for aircraft propulsion also commonly include a second turbine, known as a power turbine, located downstream (aft) of the high pressure turbine. The power turbine extracts energy from the gas stream to drive a plurality of propeller or fan blades which provide the propulsive force to move an aircraft.
A recent improvement over the engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 071,594 filed Jul. 10, 1987 now allowed, a continuation of U.S. patent application Ser. No. 728,466, filed May 1, 1985, now abandoned, a continuation-in-part of U.S. patent application Ser. No. 437,923--Johnson, filed Nov. 1, 1982 now abandoned. Johnson discloses a gas turbine engine in which first and second annular arrays of propulsor blades extend radially outward from the power turbine. The power turbine includes a plurality of arrays of turbine blades alternately coupled to first and second rotors such that the rotors counter-rotate when the gas stream passes through the power turbine. The first and second arrays of propulsor blades are coupled respectively to the first and second rotors to effect propulsive movement.
The Johnson patent application discloses first and second rotors coaxially positioned about a static structure wherein the first rotor is rotatably coupled to the static structure by a first set of roller-element bearings, and the second rotor is rotatably coupled to the static structure by second set of roller element bearings.
A disadvantage of supporting both rotors directly on the static structure is that axial bending of the static structure may cause turbine blades in one annular array to deflect, or, at worst, collide with counter-rotating turbine blades in an adjacent annular array. Such deflections may occur when the static structure experiences bending forces from the propulsor blades while also supporting the weight of the rotors. As an aircraft undergoes maneuvers or is subjected to external forces, the static structure is subjected to bending moments resulting in deflections of the rotor supports. If the rotational axis of the structure at the first set of bearings becomes significantly different from the rotational axis of the structure at the second set of bearings, adjacent arrays of turbine blades may have different axes of rotation and collide with each other. Because such a collision may cause serious damage to the power turbine, it is desirable to provide a power turbine for an unducted fan type engine in which the adjacent arrays of turbine blades are not subject to deflections of this type.
In aircraft employing unducted fan type engines, the engine mounts coupling the engines to the aircraft are attached to each engine forward of the power turbine section in order to avoid interferences with the propulsor blades. Part of the static structure is suspended rearward of the engine mounts centrally through the power turbine section in order to support the rotors of that section. A disadvantage of supporting each rotor directly on the static structure is that the static structure must be of an extended length in order to separately support each rotor. In order to provide a relatively rigid, non-deflecting support for this lengthy suspended configuration the overall size and mass of the stator must be relatively large. The corresponding weight increase directly affects aircraft fuel efficiency. If the rotors could be supported in a manner which would permit the length of the static structure to be reduced, the weight of the engine could also be reduced and the fuel efficiency of the aircraft further increased.